Oil or gas exploration often utilizes seismic analysis. Shot holes are drilled into the earth and a charge is placed in the shot hole, usually at the bottom. Detonation of the charge sets up shock waves in the surrounding earth. Instruments, typically located on the earth's surface about the shot hole, analyze the pattern of the seismic shock waves to determine underground formation structure and the potential for the presence of oil or gas.
Applicant's invention provides a new and improved method and device for sealing charges at the bottom of shot or bore holes. The present invention overcomes a problem sometimes found in conventional methods of plugging shot holes which use loosely poured bentonite, a substance that will swell when absorbing water. Typically, during the drilling process, water, dirt, and drilling lubricants, form a thick, muddy solution filling the shot hole. Most effective sealing methods would seal the charge directly above the charge at the bottom of the shot hole to allow optimum downward and outward energy dispersion when the charge is detonated. However, loosely poured bentonite and other clays are often unable to penetrate to the necessary depths to seal the charge.
Applicant's method begins with providing a container which is comprised of an expandable material, such as bentonite or a mixture of bentonite and other clays, wrapped in a water based mud suspension-soluble material, such as starch paper. The container is typically coated with vegetable oil to delay dissolution of the starch paper until the container reaches the bottom of the bore hole. The coated container is then dropped into the shot hole. Upon dissolution of the paper, the expandable material reacts with the mud suspension and expands against the walls of the shot hole. In this manner, it effectively traps the charge between the plug that is formed by the expanded clay and the bottom of the shot hole. When the charge is then detonated, the plug helps contain the energy of the detonation so more energy is concentrated downward and outward for more effective seismic readings.